As electrical systems age, defects such as cavities inside of insulating materials, thinning of insulation in motor and transformer windings, contamination across insulating surfaces, incorrect voltage to ground spacing, and so on, can begin to discharge. The presence of these electrical discharges is an indicator of hidden defects which, if left unattended, can lead to system failure. In fact, the discharges themselves will, over time, degrade the material that is sustaining them also leading to system failure. Because these discharges may occur within the interior of an insulating material and because these discharge events can be very small in absolute magnitude, their presence can be unnoticeable to human senses.
A partial discharge (PD) is a flow of electrons and ions which occurs in a gas over a small volume of the total insulation system. The defects concentrate the electric stress and can degrade the dielectric. The degradation initiates within a limited portion of the dielectric and typically does not lead to acute failure immediately. Instead, the PD in an insulation system performs as a chronic symptom which develops and eventually causes failure. The characteristics of a PD are “health indicators” which represent the performance of the insulation system in an electrical apparatus. Attempts to detect, analyze, and locate PDs have been made for a long time, including systems designed to measure and analyze acoustic energy emitted by PD pulses, optical energy emitted by PD pulses, and/or electromagnetic energy emitted by PD pulses. In electromagnetic detection systems, induction coupled (magnetic) sensors and capacitive coupled (electrical) sensors have been used to detect the PD pulses and pass a corresponding PD signal to analyzing equipment.
In electromagnetic terms, a PD pulse is characterized by a fast rise time in the range of sub-nanoseconds (sub-ns) and a signal bandwidth up to the gigahertz (GHz) range at its origin. The initial ns or sub-ns spike is followed by an “ionic portion,” which may have a longer duration (about 100 ns). In a shielded power cable, the PD signal propagates along the power cable from its origin, but high frequency components of the signal attenuate more significantly (i.e., over shorter distances) than lower frequency components.
Known systems have attempted to characterize and locate PD sources by analyzing the PD signals in the time domain (such as an oscilloscope providing an output of detected amplitude as a function of time) or in the frequency domain (such as a spectrum analyzer providing an output of signal magnitude as a function of frequency in a spectrum). Aspects of such systems are described in:                “Estimating the Location of Partial Discharges in Cables,” IEEE Transactions on Electrical Insulation, Vol. 27, No. 1, February 1992;        “The Case for Frequency Domain PD Testing in the Context of Distribution Cable,” IEEE Electrical Insulation Magazine, Vol. 19, No. 4, July/August 2003;and the references cited therein.        